The present invention generally relates to galvanic cells of the type which include a nickel-plated steel cup which functions as a current collector, a cathode which comprises and preferably essentially consists of manganese dioxide and a conductive agent pressed into the steel cup, a zinc anode, a separator and an aqueous alkaline electrolyte.
General experience has shown that alkaline manganese dioxide/zinc cells of this type have substantially poorer discharge characteristics after storage at temperatures substantially in excess of room temperature than in their freshly prepared state. In particular, losses of capacity tend to occur. The development of a contact or interface resistance between the MnO.sub.2 electrode (pressed in the shape of a ring) and the nickel substrate of the steel cup has been identified as the cause of this phenomenon.
It has been demonstrated by especially designed experiments that several factors may contribute, to differing extent, to the development of this interfacing layer. However, an exception to these factors is the alkali selected for the electrolyte since there are no specific differences between cells with pure KOH, NaOH or LiOH electrolytes in terms of their electrical characteristics.
In contrast, it has been found that manganese has a marked unfavorable effect in terms of this contact resistance. This can be concluded from the exhibited increase in the resistance of nickel plates when immersed into an alkaline MnSO.sub.4 solution and when positively polarized in the solution for a short time. In such cases, thin layers which have the assumed composition of Mn(OH).sub.x (in which x&gt;2) are formed on the nickel surface. These layers are also formed in pure MnSO.sub.4 solutions which are weakly acidic, due to hydrolysis, with such layers tending to an even greater increase in resistance.
It has further been observed that with respect to the stability of manganese dioxide during storage at elevated temperatures, polarization with an amount of current corresponding to about 2% of the total capacity of the manganese dioxide causes no additional deterioration of the loss of capacity commonly experienced. However, this tends to cause a corresponding polarization during discharge (predischarge), during which the contact resistance markedly increases.
Also recognized as an influential factor affecting contact resistance is the dissolution of metallic impurities from the manganese dioxide, which leads to an increase in resistance or the pressure which is needed to prepare the electrode. High pressures are apparently unfavorable since the prepared electrode bodies are then relatively brittle and become mechanically soft all the more rapidly during the swelling which inevitably follows.
A known measure for reducing this contact resistance deals with the current collector or take-off, rather than the electrode material, and primarily requires that the current collector be chemically inert. Thus, it is proposed in DE-PS No. 1,421,582 that the positive steel cup of a primary galvanic cell be internally plated, preferably with gold. The disadvantages of this measure are the high price of gold as well as the processes and costs involved with galvanic gold-plating.